This invention relates to a method for treatment of a net quench water stream and in particular to treatment of same stream for removal of organic contaminants prior to its disposal to the environment or its reuse for dilution steam generation required in ethylene plants.
Base petrochemicals, such as ethylene and propylene, are largely produced by steam cracking of saturated hydrocarbon (H/C) feedstocks. In the endothermic cracking process, H/C plus steam diluent are decomposed between 750xc2x0 C. and 900xc2x0 C. by fundamental free radical reactions initiated by the rupture of the Cxe2x80x94C covalent bond. Beyond the primary reaction products of ethylene and propylene, many other co-products are also produced in varying quantities including H2, paraffins, olefins, acetylenes, diolefins, cyclics, aromatic compounds and coke together with CO, CO2, H2S and a series of organic sulfur compounds. The composition of the pyrolysis effluent cracked gas (CG) varies with feedstock composition and severity of steam cracking.
The low molecular weight pyrolysis effluent products are reactive at high temperatures and will undergo further reaction to less desirable secondary reaction products unless the reaction temperature is rapidly reduced to below about 200xc2x0 C. The hot furnace CG is therefore rapidly cooled in Transfer Line Exchanger (TLE) that generates very high pressure (VHP) steam used for power production within the ethylene plant.
For gaseous feedstocks (ethane, propane and butanes), a Quench Oil Tower (QOT) is not required because only small amounts of C5+ liquids are produced. For these feedstock types, a simple Quench Water Tower (QWT) is used to cool the effluent gas from the TLE.
The CG is cooled in the bottom of the QWT to near its adiabatic saturation temperature causing condensation of tars and other heavy oily components in the CG. The CG is further cooled by contact with recirculating quench water (QW) as it flows up the QWT, thereby condensing most of the dilution steam and part of the H/C in the CG. The recirculating QW leaving the QWT carries all condensed H/C components both dissolved and separate phase in the form of tars and oils as well as coke and complex oligomers and emulsions.
Water is highly suitable for quenching purposes because it is both an effective heat transfer media and inexpensive. The employment of water in the quenching operation, however, has one great attendant disadvantage, after treating the furnace cracked gas with water, the quench medium contains significant amounts of dissolved and emulsified hydrocarbon oils, as well as heavy tar-like polymers and coke particulate matter. The oils are comprised of aromatic hydrocarbons and light polymers. These materials form stable oil/water emulsions when the cracked gas stream is intimately mixed with the quench water. The resulting emulsions comprise from about 2000 to more than 6000 parts oil per million parts emulsion. The stability of the emulsion is apparently due, at least in part, to a mutual affinity between the unsaturated hydrocarbon components in the dispersed oil phase and the continuous aqueous phase. Thus, the emulsion will resist efforts to separate it sharply into its various phases.
The QW from the bottom of the QWT is settled in an Oil-Water Separator (O/WS) that has three compartments in series separated by weirs, the heavy tar and solids is withdrawn from the 1St compartment, the raw QW from the 2nd and the light pyrolysis gasoline from the 3rd respectively.
The raw QW, from the O/WS, still contains residual fine particle solids, unsettled free oil, emulsified oil, and dissolved H/C""s. Most (90-95%) of this raw QW at 90xc2x0 C. is recirculated for low-level heat recovery in the plant before returning to the QWT. The net (discharge) raw QW is either: (1) used to generate dilution steam for steam cracking as a closed loop system, or (2) purged to battery limits as an open-loop system.
This net raw QW discharge can be pretreated to remove the residual suspended solids, and free and emulsified oil in order to prevent and/or reduce fouling in a downstream closed dilution steam generation system. On the other hand, if the excess raw water were simply purged to battery limits, it would be desirable to purify this water to such an extent that it could be discharged into local streams without causing pollution. Sufficient impurities present in the wastewater would adversely affect riverways, oceans, aquifers, fish and other wildlife.
Because ethylene plants cracking gas feedstocks do not have a QOT prior to the QWT, quench water in these plants is characterized by being more fouling service and more susceptible to emulsion formation than its counterpart in liquid cracking plants. A particular problem is the entrainment of fouling species in the quench water slipstream to the dilution steam generator (DSG).
The feed to the QWT is the furnace cracked gas. The QWT is also a dump for many other recycled streams, both continuous and intermittent, which may cause changes in the surface properties of the water as well as its pH. A low pH ( less than 4.5) or a high pH ( greater than 9.5) makes it difficult to separate the emulsified oil. In addition, a low pH raises corrosion concerns, and a high pH increases foaming tendencies and causes difficulties in oil/water separation.
Spalled coke and coke fines from furnace transient (decoking) conditions reaches the QWT, which suspends in both the oil and water phases. Tars and heavy oil in furnace effluent streams are also contained in the bottom section of the QWT. They are heavier than water and settle down. In the upper section of the QWT, lower MW hydrocarbons condense and separate as light oil. The combination of the tar, heavy oil, and polymers with the coke fines makes a gummy agglomerate that causes fouling and blockage of the trays and other internals.
Unsaturated reactive polymer precursors such as styrenes, indenes and dienes have appreciable solubility in the water phase, making them difficult to separate from the quench water using conventional separation techniques. Further, these components tend to polymerize when exposed to high temperatures encountered in downstream systems. Thus, it would solve a long felt need in the art if an effective method for removing these soluble components from the QW could be found.
In conventional systems, the condensed dilution steam/hydrocarbons and circulating quench water from the QWT are phase separated in an Oil/Water Separator. In gas crackers this separation is difficult because of small difference in specific gravity and large potential for emulsion formation. Free and emulsified oil carried with the water to the low pressure water stripper (LPWS) and dilution steam generator (DSG) contain polymer precursors that cause fouling of these towers.
To minimize heavy oil/tar carryover with the QW to the LPWS and DSG one or more of the following traditional systems has typically been used in the past:
Addition of gasoline to enhance phase separation (emulsion breaking).
Hydro-cyclone.
Filterxe2x80x94Coalescer.
Dispersed Oil Extractor (DOX) system.
Induced Gas Floatation (IGF) system.
The Dispersed Oil Extractor (DOX) system is an industrial system used to remove emulsified oil and suspended solids from the quench water. The system consists of a primary granular media coalescer filled with a multi-layer of different size granular material, followed by a vertical coalescer filled with carbon media that further coalesce the oil. The oil coalescence is finished in a horizontal performax separator containing a matrix plate section and a separation section that allows the separation of the three phases (light oil, treated QW and heavy oil). This system does not remove dissolved hydrocarbons from the QW.
Strausser et al., U.S. Pat. No. 3,507,782, describes a process for the purification of plant process wastewater by separation of dissolved and emulsified hydrocarbon from aqueous media. The dispersed phase of stable emulsions comprising aromatic hydrocarbon-containing oils in aqueous media is de-emulsified by intimately contacting the aqueous media with small amounts of aromatic hydrocarbon solvent. This results in an oil-rich phase and an emulsified oil depleted aqueous phase, and passing the oil depleted aqueous phase through a finely divided crystalline silica coalescing medium to de-emulsify the dispersed phase of the remaining emulsified oil. This system does not remove dissolved hydrocarbons from the QW.
Yoshimura et al., U.S. Pat. No. 4,336,129, describes a method for treating water-containing waste oil and solid constituents and forming a water-in-oil emulsion, which comprises adding to the water-containing waste oil, a small amount of aromatic treating oil having an aromatic content of the treating oil must be greater than that of the waste oil in the water in order to break the emulsion. The water-containing waste oils generally taught for treatment by Yoshimura et al. ""129, are typically those originating from coal tar plants, which have a large unsaturated hydrocarbon content with greater hydrophilic property than oils with reduced unsaturated hydrocarbon content. Accordingly they tend to form a water-in-oil emulsion. Solids present in these water-containing oil wastes comprise iron compounds, resinous matters comprising aromatic condensed ring compounds, coke powder, etc., swells about ten times of the volume of its dried state. The oil fraction contains mainly benzene homologues as light distillates, naphthalenes as medium distillates and tricyclic aromatic compounds such as anthracenes as heavy distillates. The specific gravity of the oil fraction is relatively close to that of water and it varies depending upon the composition of the particular oil. The specific gravity becomes smaller than water as the proportion of light distillate increases or as the temperature rises. This system does not remove dissolved hydrocarbons from the QW.
Jordan et al., U.S. Pat. No. 5,656,173, describes a method of removing dispersed oil from an oil-in-water emulsion by dissolved gas flotation. The steps involved include dissolving gas in water to form an aerated solution, and introducing the emulsion and aerated solution into a treatment vessel in which is positioned a coalescing media formed by an assembly of closely spaced corrugated plates of oleophilic material. The emulsion and aerated solution are passed in contact with the plates to cause oil droplets to coalesce on the plates. The small gas bubbles in the aerated solution adhere to the oil droplets to increase the buoyancy of the oil droplets so that the oil droplets rise more readily to the surface of the emulsion, and the accumulated oil is then removed from the surface. This system does not remove dissolved hydrocarbons from the QW.
Bibaeff, U.S. Pat. No. 4,800,025, describes a method for the dispersed gas flotation and separation of insoluble, dispersed contaminants from a liquid. The Bibaeff ""025 apparatus is comprised of a horizontal series of flotation cells, separated by baffles that permit the substantially horizontal flow of liquid from one cell to the next. Each cell is equipped with one or more gas dispersing nozzles and screens that aid in the coalescence and flotation of the contaminant particles. Also, the Bibaeff ""025 apparatus includes an inclined baffle above the horizontal series of cells to urge the floated impurities toward a weir positioned to remove the impurities from the surface of the liquid. This system does not remove dissolved hydrocarbons from the QW.
Cairo et al., U.S. Pat. No. 5,080,802, describes a method for flotation removal of suspended impurities by induced gas flotation. The apparatus induces maximum gas volumes consistent with optimum mass transfer of gas medium to suspended impurities in the liquid while controlling inter-cell or vessel chamber turbulence. The maximum gas induction without turbulence is achieved through the use of microscopic gas bubbles. Such microscopic gas bubbles provide massive surface area for the suspended impurities to adhere to and allow for utilization of an apparatus that is smaller and more compact for comparative treatment volumes. This system does not remove dissolved hydrocarbons from the QW.
Present quench water treating processes including the traditional filter/coalescer, DOX and the DGF address with some success the removal of the free insoluble oil from the quench water. All well designed units are capable of removing the free oil from about 500 wppm free oil content down to between 20 and 50 ppm free oil. None of these prior art processes, however, are capable of removing the dissolved oil that has a larger content of unsaturated hydrocarbons and polymer precursors from the QW. Because of the tendency of these components to foul downstream LPWS and DSG systems, it would represent a notable advance in the state of the art if a process that effectively removed these compounds were developed.
The present invention provides a process for removing substantially all organic material from a quench water stream, including the dissolved oils. That is, treatment of quench water in accordance with the method of this invention can reduce the content of organic contaminants to a level less than about 50 ppm, even less than about 10 ppm. Moreover, the four primary functional groups of contaminants (polymer precursors) being: conjugated dienes, carbonyls, styrenes and indenes may be reduced by this invention to concentrations approaching less than 2 ppm each. In treating a quench water solution having a quantity of organic material dissolved therein, the process of the present invention preferably provides for intimately mixing a wholly fresh or virgin water-immiscible organic extracting solvent by countercurrent flow with the quench water solution in a multi-stage liquidxe2x80x94liquid extractor at a temperature above ambient but preferably below 100xc2x0 C.
In the highly efficient extraction preferred process of the present invention, polymer precursors contaminants, dienes, carbonyls, styrenes and indenes, are removed from the quench water to a level of 2.0 ppm or less each. There is, however, a finite solubility of the organic extracting solvent in the quench water solution. To remove this content of residual organic material from the extracted quench water, the quench water as raffinate from the solvent extractor, hereinafter referred to as xe2x80x9cquench water raffinate,xe2x80x9d is subjected to steam stripping. The quench water raffinate enters the top of a steam stripping tower. The raffinate flows downward through the tower, while injected low pressure steam flows upward in the tower, and by altering the partial vapor pressure of the residual organics in the quench water raffinate, the steam removes residual organic material from the quench water raffinate stream. A pretreated quench water stream is thus provided that is substantially free of organic material and contaminants including any and virtually all monomeric polymer precursors that previously could not be removed by the prior art systems. The pretreated quench water stream then can be suitably used to generate steam in the dilution steam generation system without fouling.
The organic extracting solvent employed in the counter-current, multistage contact extraction of the quench water is a xe2x80x9cvirginxe2x80x9d extracting solvent in the entirety of its volume used. That is, with respect to any volume of solvent that first comes into contact with a volume of quench water, no portion of this solvent volume has previously been in contact with a prior portion of quench water without also having first been completely regenerated to its virgin state by distillation. In other words, each volume of organic extracting solvent supplied to the extraction column is either passed through one time only or, if reused, is first completely regenerated to the absorption capacity of a virgin extracting organic solvent. This condition is essential to achieving an essentially less than 10 ppm total concentration of dissolved unsaturated polymer precursors in the quench water raffinate. Another necessary condition to achieve this low level of dissolved polymer precursors is that the solvent and quench water must be brought into contact while each is at an elevated temperature, that is a temperature greater than 25xc2x0 C. and up to about 120xc2x0 C., preferably while each is at an initial column input temperature of from 35xc2x0 C. to about 120xc2x0 C., and more preferably at a temperature ranging from about 50xc2x0 C. to about 120xc2x0 C.
It has been found that the counter-current flow contact of a quench water stream with a water immiscible solvent of a lower density under agitation and in multiple contact stages while both fluids are at greater than ambient temperature surprisingly substantially removes from the quench water solution substantially all polymer precursor constituents that processes heretofore either did not contemplate to exist and certainly did not to any substantial extent remove from the quench water stream.
Since a preferred extracting solvent is one rich in aromatics such as benzene, toluene and/or xylenes, to the extent that the quench water solution contains like aromatic constituents as contaminants, these will not be removed by the extracting solvent, and may even be somewhat enriched in the quench water raffinate. However, because the quench water raffinate produced in the present invention is of a low and/or essentially nil content of polymer precursors, the raffinate may be subjected to steam stripping distillation without concern for fouling the steam stripper operating surfaces with polymeric materials. The quench water raffinate may be steam stripped at subatmospheric, near atmospheric or superatmospheric pressure at bottom column reboil temperatures of from about 110xc2x0 to about 130xc2x0 C. or greater to remove residual aromatic constituents and further reduce the already low level of residual polymer precursors, all of which exit in the vapor overhead product of the steam stripping column.
The steam stripped quench water raffinate taken as a bottom product from the steam stripping column, hereinafter referred to as the xe2x80x9cpretreated quench waterxe2x80x9d stream, will contain a total quantity of organic constituents which is on the order of less than 20 wppm and a quantity of polymer precursors of 10 wppm or less, generally less than about 5 wppm. The pretreated quench water is now suitable to be heated at high temperature and pressure used in the production of dilution steam without fouling the dilution steam generation equipment. The pretreated quench water, a product of the process of this invention, is free of such objections.